Fix oversight in flattening of subqueries with empty FROM.

I missed a restriction that commit f4abd0241d
should have enforced: we can't pull up an empty-FROM subquery if it's under
an outer join, because then we'd need to wrap its output columns in
PlaceHolderVars.  As the code currently stands, the PHVs end up with empty
relid sets, which doesn't work (and is correctly caught by an Assert).

It's possible that this could be fixed by assigning the PHVs the relid
sets of the parent FromExpr/JoinExpr, but getting that to work is more
complication than I care to add right now; indeed it's likely that
we'll never bother, since pulling up empty-FROM subqueries is a rather
marginal optimization anyway.

Per report from Andreas Seltenreich.  Back-patch to 9.5 where the faulty
code was added.

src/test/regress/expected/join.out

@@ -2184,6 +2184,40 @@ select aa, bb, unique1, unique1
 ----+----+---------+---------
 (0 rows)
 
+--
+-- regression test: check handling of empty-FROM subquery underneath outer join
+--
+explain (costs off)
+select * from int8_tbl i1 left join (int8_tbl i2 join
+  (select 123 as x) ss on i2.q1 = x) on i1.q2 = i2.q2
+order by 1, 2;
+                   QUERY PLAN                    
+-------------------------------------------------
+ Sort
+   Sort Key: i1.q1, i1.q2
+   ->  Hash Left Join
+         Hash Cond: (i1.q2 = i2.q2)
+         ->  Seq Scan on int8_tbl i1
+         ->  Hash
+               ->  Hash Join
+                     Hash Cond: (i2.q1 = (123))
+                     ->  Seq Scan on int8_tbl i2
+                     ->  Hash
+                           ->  Result
+(11 rows)
+
+select * from int8_tbl i1 left join (int8_tbl i2 join
+  (select 123 as x) ss on i2.q1 = x) on i1.q2 = i2.q2
+order by 1, 2;
+        q1        |        q2         | q1  |        q2        |  x  
+------------------+-------------------+-----+------------------+-----
+              123 |               456 | 123 |              456 | 123
+              123 |  4567890123456789 | 123 | 4567890123456789 | 123
+ 4567890123456789 | -4567890123456789 |     |                  |    
+ 4567890123456789 |               123 |     |                  |    
+ 4567890123456789 |  4567890123456789 | 123 | 4567890123456789 | 123
+(5 rows)
+
 --
 -- Clean up
 --

src/test/regress/sql/join.sql

@@ -365,6 +365,19 @@ select aa, bb, unique1, unique1
   from tenk1 right join b on aa = unique1
   where bb < bb and bb is null;
 
+--
+-- regression test: check handling of empty-FROM subquery underneath outer join
+--
+explain (costs off)
+select * from int8_tbl i1 left join (int8_tbl i2 join
+  (select 123 as x) ss on i2.q1 = x) on i1.q2 = i2.q2
+order by 1, 2;
+
+select * from int8_tbl i1 left join (int8_tbl i2 join
+  (select 123 as x) ss on i2.q1 = x) on i1.q2 = i2.q2
+order by 1, 2;
+
+
 --
 -- Clean up
 --